The reasons for failure of neuron regeneration and regrowth in chronic plaques, the most prevalent lesions in MS patients, are not understood. This project will test the hypothesis that the extracellular matrix (ECM) molecule breakdown, which occurs in acute MS lesions, contributes to impaired axonal regrowth in chronic lesions. The pathophysiology of ECM heparan sulfate proteoglycan (HSPG) alterations and of signaling molecules involved in neuronal outgrowth in MS will be investigated. The extent of fragmentation and cellular and topographic localization of the ECM HSPGs perlecan and agrin in different MS lesion stages, in control patient CNS samples and in neuropathologically distinct mouse MS models wilt be analyzed to determine relationships to lesion progression and specific injury patterns. The effects of enzymatically degraded ECM HSPGs on neurite outgrowth will be assessed directly in an in vitro model. The expression of ephrins (eph) and ephrin receptors (Eph), a family of neuron signaling molecules with both inhibitory and stimulatory roles in axonal growth and path finding in CNS development, will be analyzed in lesions, normal-appearing white and gray matter in MS patients, in control CNS tissues and in mouse MS models to determine relationships of expression alterations to specific neuropathologic features. Effects of catabolized ECM proteoglycans on eph/Eph expression in neurite outgrowth will also be assessed. From these combined in situ and in vitro studies, the contributions of ECM proteoglycan breakdown and eph/Eph alterations to the failure of neuron regrowth will be elucidated. The information obtained will be essential for understanding cellular and molecular mechanisms that result in the failure of endogenous repair in MS lesions and will point to new, specific therapies targeting the CNS ECM and eph/Eph. Moreover, they will be important for understanding results of stem cell-based therapies of MS lesions which require recapitulation of developmental processes that are dependent on the CNS ECM and eph/Eph signaling.